The present invention relates to balanced plug valves, and more particularly to balanced plug valves having contoured valve walls, the contour shape being alterable to result in desired flow characteristics.
Control systems and manual applications utilize various types of valves to turn fluid flows on and off, and also to modulate the rate of fluid flow through the valve. Fluid flow through a valve results from pressure differentials between upstream sources and downstream destinations. Fluid flow is a function of pressure differentials and conduit resistance. Control is generally achieved by varying the resistance to flow by varying the available flow area between zero and a maximum. A valve is the conventional method of varying area.
Sliding gate valves present one method of varying flow area. However, in such a valve, the differential pressure from the upstream side to the downstream side multiplied by the area of the obstruction separating each side results in a substantial number. This number represents a load on the guides supporting the gate. This load increases friction in a manner proportional to the area and pressure drop. With increased friction, the amount of force required to move the gate increases, thus requiring more powerful actuators. With greater actuator force requirements, costs escalate. Further, control system deadband becomes larger, which negatively affects system stability.
Plug type valves are an additional method of varying flow area. These valves reduce the flow area by forcing a plug into a hole. When the plug is lowered from the upstream side, typically the result is that the plug slams shut against a valve seat due to upstream pressure and inertia forces pushing the plug toward the hole. This slamming causes hammering which creates noise and valve damage. Forcing a plug into a hole from the downstream side can also reduce the flow area. In such a scenario, the obstruction pushes against a substantial opposing force, the force being proportional to hole size and pressure drop between the upstream and downstream sides. With increased opposing forces, the amount of force required to move the plug increases, thus requiring more powerful actuators. Again, with greater actuator force requirements, costs escalate.
In both the gate valve and plug valve instances, the difference in upstream and downstream pressures is the root of their shortcomings. To overcome these shortcomings, balancing of fluid forces is required.
One known arrangement utilizes two circular seats where the pressure forces cancel. These valves are relatively larger and more expensive than the standard gate and plug valves. Further, it is often difficult to ensure proper mechanical closure of both seats.
A second known arrangement utilizes one circular seat with a balancing chamber connected to the upstream pressure with a movable piston tied to a valve stem. These valves are complex, and again more expensive to manufacture.
As an alternative to the aforementioned larger and more costly balanced valves, it is known to create a balanced valve where the flow passes through a balanced plug that is typically in the shape of a cylinder. The cylindrical or other closed perimeter shaped plug that allows fluid to pass through is known as a balanced plug and is a key element in forming a balanced plug valve. The cylinder method successfully eliminates the friction and back pressure forces, thus forming a balanced valve. However, the known cylinder type balanced valves have their own shortcomings. These include the fact that they have poor capability for flow modulation or for tight shut-offs.
In view of the foregoing limitations and shortcomings of the above noted devices, as well as other disadvantages not specifically mentioned, there exists in the art a need for a balanced plug valve with the ability to predictably modulate flow and also provide for tight shut-off of flow.
The present invention is directed to a balanced plug valve. The valve has a valve body with at least an input port and an output port. Typically, there is a valve bonnet having a side facing an interior of the valve body. The bonnet is removably attached to the valve body at a bonnet aperture. A valve stem is slidably mounted through the bonnet or the valve wall. The valve stem is connected at a first end to an actuator at a location exterior to said valve body. The valve stem has a second end located within the interior of the valve body. A balanced plug is mounted on the interior valve stem end. At least one wall has a contour shape located on the interior. The wall is tightly sealable with the balanced plug at a closed balanced plug position. The wall is dimensioned to form a variable gap with the balanced plug, through which fluid flows, as the balanced plug is displaced. The contour shape, which determines the flow area, influences the fluid flow rate relative to a plug position or displacement.